The present invention relates to an insulation state detector which detects a ground fault or insulation state with respect to a ground potential portion on the basis of the charging voltage of a flying capacitor charged by a DC power source insulated from the ground potential portion.
For example, in a vehicle which uses electric power as energy for propulsion, it is common to insulate a high-voltage (for example, 200 V) DC power source from the vehicle body. In addition, as an insulation state detector which detects a ground fault or insulation state with respect to a ground potential portion of such a DC power source, a device that uses a flying capacitor charged by the DC power source is known in the related art. This kind of insulation state detector detects a ground fault resistor or insulation state on the positive or negative side by charging the flying capacitor with the positive or negative electric potential of the DC power source and measuring the charging voltage using a measurement section, such as a microcomputer.
In known insulation state detectors, a charging circuit, in which current flows through a negative-side ground fault resistor between a positive-side ground fault resistor and a negative-side ground fault resistor which are connected in series to each other, is formed when a flying capacitor is charged by the positive electric potential of a DC power source. On the other hand, when the flying capacitor is charged by the negative electric potential of the DC power source, a charging circuit in which a current flows through the positive-side ground fault resistor is formed. In addition, in both the charging circuits, the charging current flows through a connection point between the positive-side ground fault resistor and the negative-side ground fault resistor. The electric potential at this connection point (electric potential corresponding to the voltage division ratio of the positive-side ground fault resistor and the negative-side ground fault resistor) is equal to or smaller than the positive electric potential of the DC power source or equal to or larger than the negative electric potential of the DC power source.
Accordingly, when measuring the charging voltage in known insulation state detectors, the positive electrode of a flying capacitor connected to a measurement section is connected to the positive electric potential at the time of charging using the positive electric potential of the DC power source and is connected to the connection point between the positive-side ground fault resistor and the negative-side ground fault resistor at the time of charging using the negative electric potential of the DC power source. Such a connection change according to the state of the flying capacitor is performed by changing the on/off patterns of a plurality of switches provided in the device (for example, Patent Documents 1 and 2).    [Patent Document 1] JP-A-8-226950    [Patent Document 2] JP-A-2007-170983
In the vehicle which uses electric power as energy for propulsion as described above, in order to improve the driving efficiency of a load, the positive electric potential of a DC power source may be boosted and supplied to the load. In such a case, it is necessary to detect a ground fault or insulation state with respect to a ground potential portion for each of the primary side beforehand voltage boosting and the secondary side after voltage boosting. In order to use the above-described insulation state detector for the detection, the charging current of a charging circuit of a flying capacitor using the positive electric potential of a DC power source needs to flow not only through a negative-side ground fault resistor on the primary side but also through a negative-side ground fault resistor between a positive-side ground fault resistor and a negative-side ground fault resistor on the secondary side, which are connected in series to each other. In addition, a charging current of a charging circuit of the flying capacitor using the negative electric potential of the DC power source needs to flow through the positive-side ground fault resistor on the primary side but also through the positive-side ground fault resistor on the secondary side.
Here, the electric potential corresponding to the voltage division ratio of both ground fault resistors, which appears at the connection point between the positive-side ground fault resistor and the negative-side ground fault resistor on the secondary side, may exceed the positive electric potential on the primary side (of the DC power source) depending on the positive electric potential on the secondary side and the positive-side ground fault resistor on the secondary side. If the electric potential at the connection point between the positive-side ground fault resistor and the negative-side ground fault resistor on the secondary side exceeds the primary-side positive electric potential, in the switching state of charging a flying capacitor with the primary-side positive electric potential of the DC power source, the flying capacitor is actually charged with reversed polarity by the electric potential at the connection point between the positive-side ground fault resistor and the negative-side ground fault resistor on the secondary side which is higher than the primary-side positive electric potential.
In a known insulation state detecting circuit, however, the polarity of a flying capacitor connected to a measurement section when measuring the charging voltage is set beforehand by on/off patterns of switches. Therefore, if a flying capacitor is charged with reversed polarity as described above, the charging voltage cannot be measured by the measurement section even if a change to the state of measuring the charging voltage of the flying capacitor by the measurement section is made.
The above-described problem may be solved by connecting an insulation state detector to the secondary side. That is, if an insulation state detector is connected to the secondary side, charging of a flying capacitor using the positive electric potential is performed by the secondary-side positive electric potential which is always higher than the electric potential at the connection point between the positive-side ground fault resistor and the negative-side ground fault resistor on the secondary side. For this reason, the charging of the flying capacitor using the positive electric potential is performed with the same polarity all the time. Accordingly, the charging voltage can be certainly measured by the measurement section by deciding the on/off patterns of switches at the time of charging of the flying capacitor using the positive electric potential such that the flying capacitor is charged with the same polarity as when the flying capacitor is charged by the negative electric potential.
However, in order to connect the insulation state detector to the secondary side, it is necessary to raise the withstanding voltage of each element, which forms the insulation state detector, according to the electric potential on the secondary side. This increases the cost or the size. Moreover, in the detection of a ground fault or insulation state using an insulation state detector connected to the secondary side, it is easy for the electric potential to become unstable due to influence from an operation of a load or the like. Accordingly, it is true to try to avoid the situation if possible.